1. Field of the Invention
This invention is directed to a novel method for enhancing the quality of maize seed. In particular, this invention is directed to a method of improving maize seed vigor by defoliating maize plants after pollination. This invention also is directed to a maize seed assemblage characterized by an increased seed vigor, which can be obtained using the disclosed method.
2. Background
The quality of seed is of major concern in agriculture throughout the world. Seed quality is subject to legislative control and certification schemes in most developed countries. Forbes et al., PLANTS IN AGRICULTURE, Cambridge University Press (1992). In systems of "subsistence" agriculture, especially in the developing world, poor seed quality is a major limitation to productivity. Id.
One important factor in determining the quality of maize seed is the maturity of the seed at the time of harvest. Often, maize seed is harvested before the seed reaches optimal maturity. Reasons for premature harvest include frost risk, production plant capacity, and the need for time to move seed from a site of production in the southern hemisphere to a planting site in the northern hemisphere. In addition, many varieties of inbred lines of maize do not mature in a uniform fashion throughout the field, resulting in a seed harvest containing poor quality seed.
Seed quality is typically measured by standard germination tests. Delouche et al., Proc. Assoc. Off. Seed Anal. 50:124 (1960); Woodstock, Seed World 97:6 (1965). While the results of a standard germination test may correlate well with field emergence when soil conditions are favorable for rapid emergence, the test cannot predict field performance when soil and environmental conditions impose stress on the germinating seeds, as often occurs with early planting. Tekrony et al., Crop Sci. 17:573 (1977).
The concept of "seed vigor" evolved to compensate for the failure of standard germination tests to predict field performance under conditions of environmental stress. Adegbuyi et al., J. Agron. & Crop Sci. 161:171 (1988). Seed vigor is a reflection of those properties that determine the potential for rapid, uniform emergence of plants and the development of normal seedlings under a wide range of field conditions. 1983 SEED VIGOR TESTING HANDBOOK (Assoc. Official Seed Analysts). Seed vigor is thought to influence not only emergence and emergence rate but also plant height, stem diameter, shoot dry matter, leaf length and leaf width. Adegbuyi et al. J. Agronomy & Crop Science 161:171 (1988).
The quality of seed vigor is particularly important where a crop is grown near the limits of its climatic tolerance range or sown into adverse soil conditions. In the northern U.S. corn belt and in Europe, maize is generally planted in the early spring into soils that are, or may become, too cold and wet for optimal germination. Thus, there is a need for a method to increase the quality of maize seed, as measured by seed vigor.
There has been extensive study of the effects of crop defoliation on yield and seed quality. See, e.g., corn: Hicks et al., Agronomy J. 69:387 (1977), Tollenaar et al., Can. J. Plant Sci. 58:207 (1978), Crookston et al., Crop Sci. 18:485 (1978), Johnson, Agronomy J. 70:995 (1978), Hunter et al., Crop Sci. 31:1309 (1991); sunflower: Prokof'ev et al., Prik. Biokhimiya Mikrobiol. 8:402 (1972); soybean: Mesa et al., Crop Sci. 24:847 (1984), Vieira et al., Crop Sci. 32:471 (1992); sorghum: Onofre et al., Agrociencia 65:253 (1987); lima bean: Coggin et al., J. Econ. Entomol. 73:609 (1980). See also Saleh et al. in 1974 YEARBOOK, Indonesian Central Institute for Agriculture (rice, maize, sorghum, groundnut and soybean). The overall tenor of this body of literature is that defoliation, while unavoidable and deleterious in certain contexts, actually may have an advantageous effect, for example, with regard to increased oil content/yield of sunflower seeds. Prokof'ev et al., supra.
Moreover, Crookston et al., supra, report that defoliation can enhance yield in short-season maize hybrids. Crookston et al. found that, although leaf removal during silking consistently led to a reduction in maximum yield, defoliation at a very early growth stage, prior to flowering, enhanced yield. It has not been possible, however, to predict whether defoliation prior to flowering could provide enhanced seed vigor, since yield and seed vigor are not clearly correlated in maize. Adegbuyi et al., J. Agronomy and Crop Science 162:10 (1989) .